Skill: Find missing side lengths in perimeter problems

Perimeter is the total distance around a shape. To find a missing side:

1. Add the lengths of all the known sides.
2. Subtract that sum from the perimeter.
3. Write the answer with the correct unit.

For a rectangle, opposite sides have equal lengths.

Example: A rectangle has a perimeter of $26\text{ cm}$. Three sides are labeled $8\text{ cm}$, $8\text{ cm}$, and $5\text{ cm}$. Find the missing side.

First, add the known side lengths:

$$8+8+5=21\text{ cm}$$

The known sides make up $21\text{ cm}$ of the perimeter. Subtract from the total perimeter:

$$26-21=5\text{ cm}$$

So, the missing side is $5\text{ cm}$.

Check: The rectangle’s side lengths are $8$, $5$, $8$, and $5$.

$$8+5+8+5=26\text{ cm}$$

Remember, perimeter measures the distance around a shape. Area measures the space inside a shape.